1. Field of the Invention
The present invention is directed generally to the reaction that occurs between glucose and proteins, and more specifically to the identification of a new group of compounds which form during nonenzymatic browning and whose existence became apparent by the studies of the inhibition of this reaction by sulfites.
2. Description of the Prior Art
The reaction between glucose and proteins has been known for some time. Its earliest manifestation was in the appearance of brown pigments during the cooking of food, which was identified by Maillard in 1912, who observed that glucose or other reducing sugars react with amino acids to form adducts that undergo a series of dehydrations and rearrangements to form stable brown pigments, Maillard, L. C. (1912) C. R. Acad. Sci., Vol. 154, pp. 66-68.
In the years that followed the initial discovery by Maillard, food chemists studied the hypothesized reaction in detail and determined that stored and heat treated foods undergo nonenzymatic brown as a result of the reaction between glucose and the polypeptide chain, and that the proteins are resultingly crosslinked and correspondingly exhibit decreased bioavailiability. Finot, P. A. (1982) in Modification of Proteins, eds, Feeney, R. E. and Whitaker, J. R., American Chemical Society, Vol. 198, pp. 91-124, Washington, D.C. At this point, it was determined that the pigments responsible for the development of the brown color that develops as a result of protein glycosylation possessed characteristic spectral properties; however, the chemical structure of the pigments had not been specifically elucidated.
The reaction between reducing sugars and proteins discussed above was found in recent years to have its parallel in vivo. Thus, the nonenzymatic reaction between glucose and the free amino groups on proteins to form a stable 1-amino-1-deoxy-2-ketosyl adduct, known as the Amadori product, has been shown to occur with hemoglobin, wherein a rearrangement of the amino terminal of the beta-chain of hemoglobin by reaction with glucose, forms the adduct known as hemoglobin A.sub.1c. This reaction was also found to occur with a variety of other body proteins, such as lens crystallins, collagen and nerve proteins. See, Bunn, H. F., Haney, D. N., Gabbay, K. H. and Gallop, P. H. (1975) Biochem. Biophys. Res. Comm. Vol. 67, pp. 103-109; Koenig, R. J. Blobstein, S. H. and Cerami, A. (1977) J. Biol. Chem. Vol. 252, pp. 2992-2997; Monnier, V. M. and Cerami, A. (1983) in Maillard Reaction in Food and Nutrition, ed. Waller, G. A., American Chemical Society, Vol. 215, pp. 431-448; and Monnier V. M. and Cerami, A., (1982) Clinics in Endocrinology and Metabolism Vol. 11, pp. 431-452. Moreover, brown pigments with spectral and fluorescent properties similar to those of late-stage Maillard products have also been observed in vivo in association with several long-lived proteins, such as lens proteins and collagen from aged individuals. An age related linear increase in pigment was observed in human dura collagen between the ages of 20 to 90 years. See, Monnier, V. M. and Cerami, A. (1983) Biochem. Biophys. Acta., Vol. 760, 97-103 (1983); and, Monnier, V. M. Kohn, R. R. and Cerami, A. "Accelerated Age-Related Browning of Human Collagen in Diabetes Mellitus", (1983) Proc. Nat. Acad. Sci. 81, 583-7. Interestingly, the aging of collagen can be mimicked in vitro by the crosslinking induced by glucose; and the capture of other proteins and the formation of adducts by collagen, also noted, is theorized to occur by a crosslinking reaction, and is believed to account for the observed accumulation of albumin and antibodies in kidney basement membrane. See, Brownlee, M., Pongor, S. and Cerami, A. (1983) J. Exp. Med., 158, 1739-1744 (1983).
In general, little detailed information is available on the chemistry of the late-stage Maillard process, and it has been correspondingly difficult to determine the relation if any, that nonenzymatic browning may bear to the structural and functional changes in tissues that occurs during the aging process, and has likewise been observed to occur at an accelerated rate in individuals suffering from diabetes mellitus. Accordingly, a need therefore exists to identify to the extent possible, the structure and activity of the end products of advanced protein glycosylation, to aid in identifying the occurrence of advanced glycosylation, and to serve as an exploratory tool for the development and test of possible agents and corresponding treatments designed to retard or inhibit this form of protein aging. 2-Furoyl-4(5)-2(furanyl)-1H-imidazole has been isolated from the acid hydrolysates of browned proteins and is believed to be a cross-linker from the non-enzymatic browning of proteins, Pongor et al., Prod. Natl. Acad. Sci. USA, 81, 2684 (1984), and U.S. Pat. No. 4,665,192 (1987) and U.S. Ser. No. 885,967, filed Jul. 15, 1986 and entitled "Methods and Agents for Measuring Protein Aging".
Methods of inhibiting the Maillard reaction in vivo using aminoguanidine are known, Brownlee et al., Science, 232, 1629 (1986), and U.S. Ser. No. 798,032, filed Nov. 14, 1985, and entitled "Method and Agents for Inhibiting Protein Aging". In the food industry, sulfites were found years ago to inhibit the Maillard reaction and are now commonly used in processed and stored foods. Recently, however, sulfites in food have been implicated in severe and even fatal reaction in some asthmatics. As a consequence, the sulfite treatment of fresh fruits and vegetables has recently been banned. The mechanism for the allergic reaction is not known. Due to the severity of the possible reaction, it is imperative for sulfite-allergic subjects to know when a foodstuff has been so-treated.